In general, a mobile communication system is configured to monitor a synchronization state between a radio base station and a mobile station. For example, in a mobile communication system of the WCDMA scheme, the following two indexes are defined to determine the downlink synchronization state of a physical layer.                DPCCH (Dedicated Physical Control Channel) quality        CRC (Cyclic Redundancy Check) result        
Here, the DPCCH quality corresponds to a reception quality of pilot symbols or transmission power control (TPC) bits, for example, an SIR (signal-to-interference power ratio) or a reception level, and the CRC result corresponds to a block error rate.
A determination interval used to determine the downlink synchronization state of the physical layer described above is, for example, 160 ms. As described below, the determination for the radio link failure (RLF) state of the RRC layer is performed based on the determination result of the downlink synchronization state of every 10 ms.
That is, in the mobile communication system of the WCDMA scheme, the determination for the radio link failure state of the RRC layer based on the downlink synchronization state reported from the above-described physical layer is defined and the operation of the mobile station UE in the radio link failure state is also defined.
The determination for the radio link failure state of the RRC layer described above and the operation of the mobile station UE in the radio link failure state will be described with reference to FIG. 14.
When continuously receiving the report of “Out-of-sync”, that is, “synchronization state: NG” N313 times from the physical layer in step S102 (YES in S102), the mobile station UE starts a timer T313 (S104). Steps S102 and S104 may be performed only when the timer T313 is not activated.
On the other hand, when continuously receiving the report of “In-sync”, that is, “synchronization state: OK” N315 times from the physical layer in step S106 (YES in S106), the mobile state UE stops the timer T313 (S108). Steps S106 and S108 may be performed only when the timer T313 is activated.
When the T313 expires (YES in step S110), the process proceeds to step S112. When the T313 does not expire (NO in step S110), the process returns to step S102. Step S110 may be performed both when the T313 is activated and when the T313 is not activated.
Steps S102 to S110 may be always performed when a radio base station eNB and the mobile station UE are in a connection state. Alternatively, steps S102 to S110 may be performed every 10 ms when the radio base station eNB and the mobile station UE are in the connection state. Here, 10 ms indicates one radio frame. In addition, the connection state may be an RRC_Connected state.
In step S112, the mobile station UE regards the communication state between the radio base station eNB and the mobile station UE as a radio link failure state.
The radio link failure state may be called an RLF (radio link failure) state and refers to a state in which a communication quality between the radio base station eNB and the mobile station UE considerably deteriorates and thus communication is not possible.
In step S114, the mobile station UE clears the communication configuration between the radio base station eNB and the mobile station UE.
In step S116, the mobile station UE performs connection re-establishment processing. Here, the connection re-establishment processing may be called “Cell Update processing”. The mobile station UE may perform the above-described connection re-establishment processing, when there is no processing being performed or when an operation in the radio link failure state is not defined in the processing being performed.
In such a case, the mobile station UE performs the operation in the radio link failure state during the processing being performed, when there is processing being performed and when the operation in the radio link failure state is defined in the processing being performed.
Here, the parameters such as T313, N315, and N313 described above correspond to a hysteresis or the number of protection steps and are used to determine the radio link failure state with high accuracy and at an appropriate timing.
However, the mobile communication system adopts discontinuous reception (DRX) control, in order to reduce the power consumption of the mobile station UE, that is, for battery saving purpose.
For example, in a mobile communication system of the LTE (long term evolution) scheme which is the next generation of the WCDMA scheme, the discontinuous reception control is performed, while the radio base station eNB and the mobile station UE are connected to each other and when there is no data to be communicated, and the mobile station UE in the discontinuous reception state is configured to receive a downlink control signal transmitted via a physical downlink control channel (PDCCH) periodically, that is, discontinuously.
In such a case, since the mobile station UE has only to receive the downlink control signal transmitted via the physical downlink control channel (PDCCH) discontinuously, instead of all timings, the power consumption of the battery can be reduced.
In more detail, as shown in FIG. 15, the mobile station UE is configured to receive the downlink control signal transmitted via the physical downlink control channel (PDCCH) only at the reception duration (5 ms in the example of FIG. 15) set in each DRX cycle (1280 ms in the example of FIG. 15) and other transceivers are configured to be turned off. As a consequence, the power consumption of the battery of the mobile station UE can be reduced. The reception duration may be called On-duration.